Method and apparatus for the control of a weight suspended from a floating vessel

ABSTRACT

The invention concerns the control of the suspended weight from a vessel subject to wave and tidal action and particularly a vessel employed in connection with submarine drilling operation. The particular improvement concerns the modulation of the liquid in a hydraulic-pneumatic suspension system subject to heave action so as to maintain the volume of the gas in the accumulator substantially constant during each portion of the cycle action of the heave. This may be accomplished by adding liquid to the accumulator during one portion of the heave cycle and withdrawing liquid from the accumulator during another portion of the heave cycle.

United States Patent Larralde et a1.

METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDED FROM A FLOATING VESSEL Inventors: Edward Larralde; Glen Robinson,

both of Santa Barbara, Calif.

Assignee: Vetco Offshore Industries, Inc.,

Ventura, Calif.

Filed: June 27, 1973 Appl. No.: 373,968

Related US. Application Data Continuation-in-part of Ser. No. 274,880, July 25, 1972.

US. Cl. 175/5, 175/27 Int. Cl E2lb 19/08 Field of Search 175/5, 27, 7; 166/.5;

3,687,205 8/1972 Mori 175/27 3,714,995 2/1973 Hanes. l 175/5 3,721,293 3/1973 Ahlstone 166/.5 3,779,135 12/1973 Sugimura 60/413 Y 3,793,835 2/1974 Larralde 175/27 Primary ExaminerFrank L. Abbott Assistant ExaminerRichard E. Favreau [57] ABSTRACT The invention concerns the control of the suspended weight from a vessel subject to wave and tidal action and particularly a vessel employed in connection with submarine drilling operation. The particular improvement'concerns the modulation of the liquid in a hydraulic-pneumatic suspension system subject to heave action so as to maintain the volume of the gas in the accumulator substantially constant during each portion of the cycle action of the heave. This may be ac complished by adding liquid to the accumulator during one portion of the heave cycle and withdrawing liquid from the accumulator during another portion of the heave cycle.

16 Claims, 3 Drawing Figures METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDED FROM A FLOATING VESSEL This application is a continuation-in-part of Application Ser. No. 274,880, filed July 25, 1972.

BACKGROUND OF THE INVENTION This invention relates to improvements in methods and apparatus for the control of a suspended weight from a floating vessel and is particularly directed to the control of forces imposed on the drill string of a floating vessel employed in drilling, coring, running casing, reaming, cementing, testing, or other services in bore holes drilled in subaqueous environments where the vessel is subjected to wave or tidal action.

The particular and preferred object of this invention is to improve the operation of such systems in which the element constituting the load is suspended from a pneumatically controlled hydraulic system. As is well known in the petroleum industry, the drill string, due to the great length above the drill collar, is a very flexible member subjected to considerable stretch due to its length and due to its own weight. The weight on the bit is less than the total static weight of the drill string because of the tension in the drill string imposed by the drilling lines as is well understood by those skilled in this art. The practice during drilling is to keep the drill pipe above the drill collar in tension. The drill collar acts as a weight-producing element which exerts the load on the drill bit.

In the hydraulic-pneumatic systems of the prior art, this is accomplished by a gas pressure in an accumulator which pressurizes liquid in a hydraulic cylinder underneath the piston which supports the weight. The cylinder may be positioned to support a crown block or connected to the traveling block conventional in drilling derricks.

Wave action imposes a vertical oscillatory motion on the vessel which is imposed on an hydraulic cylinder resulting in variations in the tension in the drill pipe and therefore in variation in the load imposed upon the drill bit, when this is employed or any other load connected to the piston rod. In the case of the hydraulicpneumatic system, the pressure on the liquid underneath the piston rod is maintained by gas pressure in an accumulator; such systems are shown in the I-Ianes et al., US. Pat. No. 3,7l4,995 and in the Larralde et al. US. Pat. No. 3,7l8,3l6.

Experience with such systems has shown that a variation of about i 2% to i 5% of the suspended weight may be experienced at each wave cycle, even when no drilling advance is maintained. With drill advancing during drilling, an additional weight variation may occur. The results of these motions are that the pressure in the cylinder fluctuates and the degree of fluctuation increases as the drilling progresses.

Floating vessels operating as drilling vessels in the open sea may experience vertical motions, i.e., heave due to wave action ranging, for example, from as low as 2 inches to 20 feet or more trough to crest as, for example, has been experienced in drilling of the North Sea. However, under ordinary conditions, the ships are on station and drilling when the heave is not more than about to feet. The wave action imposes a vertical displacement of the drilling vessel at a substantially sinusoidal frequency. The period of such cycles has been reported in the range of 8 to 16 seconds but may be either greater or less. The demand on the string will vary, depending on th services which they are to provide. Thus, for landing casing or instrument survey, it is desirable to hold the piston fixed in space at the desired level in order that the casing or instrument is not subject to displacement.

There is also another circumstance where it may become important that the piston be maintained at a relatively stationary position in space. Thus, when the well suddenly develops a high pressure due to the production of gas and it becomes necessary to close the blowout preventer, it becomes highly important that the drill string remain fixed in space and not oscillate in the closed blow-out preventer so as not to damage it.

During drilling, however, the drill is to be advanced at a controlled rate while maintaining a controlled weight on the bit. It is desirable to maintain a desired upper load limit on the bit in order that excessive stresses and torque are not developed which will be so large as to injure or even cause rupture of the drill pipe. On the other hand, it is desirable that the load on the bit be not reduced excessively so that the rate of advance is unreasonably reduced. Since the cost of operation of the drilling operation is materially affected by the drilling rate, it is desirable that the drilling rate be maintained at as high a rate as is consistent with safety. This is established by the driller based on his experience and the performance of the drilling operation under consideration. The driller sets the load required for' the drilling advance to make the advance be at a satisfactory rate consistent with safety.

In the prior art, it has been suggested that the hydraulic cylinder be mounted on the derrick to support the crown block or between the traveling block and the hook and that a force be applied to the piston sufficient to maintain the desired fraction of the total load of the drill string during drilling operation so as to hold the desired load upon the drill bit.

STATEMENT OF OUR INVENTION In our preferred embodiment, the weight, which in our presently preferred applications of our invention may be a drill string, is connected as above to the piston of an hydraulic cylinder with the liquid under the piston in communication with a pneumatic accumulator under gas pressure. It is the purpose of our invention to maintain a desired pressure in the cylinder to be main tained during the complete wave cycle so that a predetermined load or a predetermined load variation on the drill can be maintained notwithstanding the amplitude or frequency or changes in the frequency or amplitude of the wave action. Where it is desired that the drill be advanced, our invention will provide a control for the advance of the bit at a desired rate or rate change in addition to the controls during nondrilling conditions.

We accomplish this objective to maintain a substantially constant pressure by modulating the volume of the liquid in the hydraulic system and the liquid in the pneumatic accumulator of the system to compensate for the tendency of the piston to move relative in space and relative to the movement of the cylinder due to wave action. We may control the pressure to hold the pistons relatively fixed in space as when it is not desired to advance the drill string, but we may also while maintaining the pressure so modulate the volume of the liquid in the hydraulic system where an advance of the piston in space, as where a downward motion of the piston as in drilling, is to be accomplished. In our preferred embodiment of a pneumatic-hydraulic system which supports a weight and is subject to cyclic action, we introduce and remove from the hydraulicpneumatic system a volume of liquid in cycles of operation sufficient to maintain a volume of gas in the pneumatic system at a substantially constant volume and pressure.

It is another object of our invention, in a pneumatichydraulic system, which is subjected to cycles of vertical displacement and which supports a weight to maintain a substantially constant force on said weight by maintaining a substantially constant volume of liquid in the pneumatic accumulator by subtracting or adding liquid to the hydraulic system in cycles of operation.

We accomplish our objective by obtaining a signal responsive to the force on the piston in the cylinder by comparing said signal with the signal corresponding to a standard which is responsive to a desired force on the piston and thus obtain an error signal, and we add or withdraw liquid from the hydraulic system to reduce or ideally cancel the error signal.

DETAILED STATEMENT This invention will be further understood by reference to the drawings of which:

FIG. 1 is a somewhat schematic showing of the arrangement of the relations of parts of the system of our invention.

FIG. 2 is a section of a conventional valve employed in our system.

FIG. 3 is a schematic diagram of a control system of our invention.

As has been explained above, the motion of the piston with respect to space is a combined motion of the piston due to the heave of the vessel and the advance of the drill during drilling. The hydraulic cylinder and piston employed in our invention may be the cylinder and piston design described in the aforementioned patents or any other suitable single or multiple cylinder and piston. We prefer, however, to employ the cylinder and piston described and shown hereinbelow.

In order to modulate the volume of the liquid in the pneumatic accumulator to maintain the volume of gas substantially constant, we establish as a reference a signal which is proportional to the pressure in the hydraulic cylinder and thus the tensile force on the drill string and the consequent strain on the piston rod and thus to the weight suspended therefrom. The magnitude of the signal parameter which is the reference is that which it is desired that the pressure should be during the phase of the operation which it is desired to be controlled.

We provide a reservoir or reservoirs of liquid from which or to which we can pass liquid to or from the pneumatic accumulator to modulate the liquid in the pneumatic-hydraulic system so as to maintain a constant pressure in the system. This we accomplish by maintaining a constant volume of liquid in the accumulator so as to maintain a constant volume of gas in the accumulator.

The signal which informs our system to add or subtract liquid from the system and the quantity thereof is a deviation of the force signal from the reference signal. If this difference is in a direction indicating that the pressure in the system is falling as a result of a transfer of liquid from the accumulator, our system will supply liquid to reestablish the volume of liquid in the accumulator. If the pressure rises above that at which the reference signal is established, the system will withdraw liquid from the accumulator to re-establish the reference pressure.

We may use any hydraulic cylinder of design to permit the support of the piston in the cylinder by liquid pressurized by a pneumatic accumulator, such as has beenheretofore referred to and to support a weight from a floating vessel on which the system is mounted. In our preferred embodiment, however, we employ a duplex hydraulic cylinder which is the subject of the aforesaid copending application, which is hereby incorporated by this reference. In a cylinder of this character, the piston is supported by two concentrically mounted cylindrical elements which move together; and the total force on the piston is the sum of the forces in each of the cylindrical elements. The elements are so arranged that on displacement in space one of the cylindrical elements increases in volume and the other increases in volume. The volume of liquid required to be added or to be subtracted to compensate for a given volume change is thus the sum of the volume changes in each of the cylindrical elements and proportional to the effective areas of the piston in each of the cylindrical elements.

FIG. 1 shows the application of a control of our invention to an operation from a floating vessel 1 acting as the drilling platform. The conventional derrick 2 mounted on the vessel carries the split crown block 3 from which the sheaves 5 are suspended by the drilling lines. The sheaves 5 carry a cylinder 4, vented at 7, in which is positioned a piston 8 connected to a tubular piston rod 9 from which is suspended the conventional hook 10 which carries a swivel 11 and the kelly 12. The drill pipe 14 is connected to the kelly and to the drill pipe which is connected to the bit 15. The casing 16 is composed of the conventional marine riser and the bore hole casing assembly together with the usual drilling equipment.

The hollow piston rod 17 passes through a seal in the piston 8 and carries a piston head 13 connected by a closed-end tube 17 to the cylinder head. The tubular member 17 is bored at 6 to provide a communication between the inlet 19 to member 17 and to annulus 18 between the tubular rod and the tubular member 17. This cylinder and rod construction and its use as a weight control have been described in the aforesaid application, Ser. No. 274,880. The tubular member 17 is connected by a pipe 19 via the solenoid valve 20, to be more fully described below, to the reservoir 21 through valves 22, pump 24, and valve 23. Valves 23 and 22 are solenoid controlled as will be described below. The reservoir 21 is also in communication with the accumulator 34 through the valve 27. The pump may circulate via line 28 and valve 29.

Instead of connecting the accumulator 34 to accumulator 30 via valve 34, we may use instead of 34 a precharged accumulator containing gas at a selected pressure less than the minimum pressure to be attained in operation. Valve 27 may be, but need not be used. A

be established in 34. The orifice is of such character and of such time constant that for the period of the heave and the pressure differences which are effective across the orifice, the pressure in the accumulator 34 remains substantially constant.

FIG. 2 illustrates a proportional metering valve by which flow proportional to the magnitude and sign of the electric signal is obtained to maintain the liquid in the accumulators 30 and 30a substantially constant.

A torquing armature 47 is supported by a flexure tube 48 in such a manner that energizing either coil 49 or coil 50 will cause torquing armature 47 to move in a clockwise or counterclockwise direction as viewed in FIG. 2, moving element 51 and deflecting spring 52 with respect to reference pin 53. The resultant movement of element 51 with respect to nozzles 54 thus varies pressure in chambers 55 causing valve spool 56 to move in the direction of lower pressure. Movement of spool 56 to the left permits flow from port 57 to chamber 58 and chamber 59 and out through port 60.

Movement of the valve spool 56 to the right connects flow port 57 with 58 and through 61 to port 62 which are interconnected with 62.

Feedback shaft 53 engages feedback spring 52 which, in turn, bears on element 51 attenuating the movement of element 51 so that movement of element 51 represents the summation of forces resulting from the relatively elasticity of flexure tube 48, feedback spring 52, and the magnetic flux forces in coil 49 or 50, thus assuring a displacement of valve spool 56 in such a manner to allow flow proportional to the electric signal to coils 49 and 50.

The valve 20 described herein is a well known valve, and no invention is claimed for the valve apart from its use in the combination and for the purpose of our invention. Other valves to regulate the direction and magnitude of flow which will function similarly in our invention may be used.

The pneumatic accumulator 30 is in communication with the cylinder 4 valve 26 being open.

The line 19 is connected to the port 57. The line 31 is connected to the port 60. A by-pass 31a connects line 31 to 19 and the accumulator 34 is connected to line 31 via valve 27 and to line 32 via valve 27a. Pump pressure is exerted at valve 22. The valves 22, shunted by by-pass valve 220, and 23, shunted by by-pass valve 23a, are positioned in lines 31 and 32 intermediate the by-pass valves and 29. Line 32 is connected to the port 62 and to the reservoir through valve 23.

With manual switches 71, 72 and 73 (FIG. 3) open, pressure is established in the gas space of and 30a from a source of gas 33 under pressure. The pump 24 or source 24', valves properly adjusted, circulates liquid through valves 22a, by-pass valve 27a and valve 23a to the reservoir. Liquid is introduced into 34 via valve 34 and in the accumulator 30 via valve 26 and into accumulator 30a via valve 30b, and cylinder 4 and annulus 18 via valve 31a. The liquid volumes and pressures are established by the pump 24, by-pass manual valves 22a, 23a, 27a and 31a suitably adjusted with the load suspended, until the strain gauge mounted on the piston tubular connecting rod 9, with the load suspended, reports a strain equivalent to the load desired to be imposed upon the piston. The valve operation is described below. The pressure to produce the desired pressure is sensed by the pressure sensor 36 and which should also be that sensed by sensors 40 and 41. Manual switches and valves are closed.

A pressure sensor 40 is mounted on the line connecting accumulator 30 to the cylinder 4 to be responsive to the pressure in the accumulator. A pressure sensor 41 is mounted on line 19.

The stress and pressure sensors are provided with readouts which produce a voltage proportional to the parameters to which they respond.

Under stationary conditions, i.e., with the vessel not subject to heave action, the system is in the following condition:

The schematic block function diagram, FIG. 3, illustrates the servo control of the volume of the liquid in the accumulator. All electrical elements used in the system are conventional, and their selection will be understood by those skilled in the art to which they pertain. Their assembly in combination with the system here described illustrates the preferred embodiment of the control assembly of our invention.

The output voltage e, of the pressure sensor 36 is compared with the output voltage e of pressure sensor 41 in comparator 42 to give an output proportional to the difference between e, and e The voltage output e of the pressure sensor 40 is compared with the voltage output e of sensor 41 to give a voltage porportional to their difference. The voltage output e, of the sensor 36 is compared with the voltage output e of the pressure sensor 40 in the comparator 44 to give a signal proportional to the difference in the voltage outputs e and e;,. The voltage output e, is compared in comparator 45 with the voltage output e of the sensor 35 to give a signal proportional to the difference in voltage e, and e,. When the pressures to which the pressure sensors are responsive are all the same, these differences are set to be zero. When the pressure in the cylinder is at the desired level, the signal resported by the comparator 45 is also zero.

The comparator may be any conventional device to give a signal responsive to the difference of two voltages, such as a summation resistance network or a differential amplifier, or a bridge.

The pump 24, with valve 29' and 29 open and valve 22 closed, circulates liquid around the reservoir against the valve 22 to establish the required fluid pressure at the valve 22 sufficient to pass liquid into the hydraulic system on demand. The valve 29 is regulated to hold a desired pressure at valve 22. Having established the voltage e,, which is the output of the pressure transducer 36, valves 34' and 27 are closed, thus trapping liquid in 34 under the established pressure to act as a fixed reference, if used instead of the orifice herein described.

Instead of a pump 24 and circulating line through the valve 29, valve 29 may be closed and liquid under the desired pressure drawn from an accumulator 24' under requisite gas pressure. When the accumulator has been sufficiently depleted in liquid, it may be recharged by pump 24 via valve 29a.

Assuming the pressure cylinder 4, annulus 18, and accumulator 30 to be that of the pressure in 34, the voltage output of the sensor 41 (e:) and 40 (2 will be equal to that of the sensor 36 (2,). Voltage e, e and e e and, therefore, e e as reported by the voltage comparator 42, 43, and 44 will be 0. At this posture, the output e, of the strain gauge 38 should be balanced against e so that e, e, 0 and the output of the comtion and switch 65 is closed to activate the solenoid 49 to hold the ports 60 and 57 to be interconnected. This will interconnect the annulus 18, cylinder.4 and the accumulator 30.

Assume that the system is mounted on a floating vessel and moves upwards as the heave starts. The cylinder moves upward with respect to the piston; and the volume in the cylinder 4 decreases; and the volume in the annulus 18 decreases. The liquid displaced by the cylinder movement is introduced into the accumulator decreasing the gas'volume and thus increasing the pressure in the accumulator and consequently the pressure in the cylinder 4 and in the annulus 18. The cylinder and the annulus is now above the pressure in 34. The voltage outputs of the sensors are thus: e and e are greater than e The voltage difference closes switch 68 and switch 67 is open. Valve 22 is closed and valves and 23 are in open position.

The unbalance in the voltages e e or e (2 causes switch 65 to be closed and switch 66 to be open activating solenoid 49 and deactivating solenoid 50. The spool 56 is moved so that ports 60 and 57 are connected, the annulus 18 is interconnected with the cylinder 4 and the accumulator 25. During this period, the annulus l8 and piston 4 and accumulator are connected to the reservoir through the valves 25 and 23.

Liquid will be displaced from the cylinder and the annulus to the reservoir. When the pressure in the cylinder, annulus and accumulator 30 are equalized and equal to the pressure in the accumulator 30, e, e e and e are all equal, and switches 67 and 68 are opened, and both 49 and 50 are de-energized. Valves 22, 25 and 23 are closed. Spool 56 moves to close ports 60 and 62.

When the cylinder has reached the crest of the wave and starts to descend the volume in the cylinder 4 increases and the volume in the annulus 18 increases. Pressure falls in the cylinder and in the annulus 18 are compared to the pressure in 30. The voltages e;, e e,. Switch 66 is closed and solenoid 50 is energized, connecting port 57 and 62. Switch 67 is closed opening valve 22 energizing solenoid 71. The solenoid 70 is designed so that it will not under these conditions close switch 68 being suitably polarized. Valve 22 is opened and valve 23 remains closed. This condition remains during the entire downward motion of the cylinder, until the pressure sensor output e equals that at 2 and also 2 switch 67 opens and switch 68 remains open. Valve 22 closes and valve 23 and 25 remain closed holding the pressure in the accumulator constant.

We prefer to employ a differential amplifier-rectifier 45a. ln such case valve 23 may remain open, switch 68 may be omitted. The inputs to the differential amplifier are the outputs e and e and the outputs of the differential amplifier-rectifier are applied, one to the coil 49 and the other to the coil 50. The differential tractive effort of 49 and 50 is, therefore, proportional to the respective signs and magnitudes of e and e The resultant displacement of the spool 56 is, therefore, proportional to this difference. The orifices at ports 65 and 61 will depend on the aforesaid difference. The rate of addition or removal of fluid from the annulus and the cylinder is thus made proportional to the demand in order to establish the desired level of forces. The excess liquid circulating through valve 23.

The system thus withdraws fluid from the system to the reservoir during the period of the heave from the trough to the crest and adds fluid to the system during the period of the heave from the crest to the trough in an amount to maintain the volume of liquid in the accumulator substantially constant. The withdrawal or the addition is interrupted when the pressure in the accumulator has reached a predetermined pressure at which the piston is to be supported, and this operation will occur even though the descent of the piston during drilling operation occurs. The criterion for the addition or withdrawal of liquid is the deviation of the pressure from a predetermined norm, which is the pressure desired to be maintained under the piston under the conditions which it is sought to maintain the piston.

Should it be desired to change the conditions to adjust the pressure, the gas pressures are adjusted to either increase or decrease the pressure in 30 and 3021 as desired; and the system will automatically adjust itself to that pressure as will be evident from what has been described above.

The following example is for the purpose of illustrating the principles of our invention and not to be understood to be any limitation thereof. It is assumed in this example that the system is tight and no loss of liquid from the hydraulic-pneumatic system occurs and the total volume of liquid in the accumulator, cylinder and connecting lines is constant.

The vessel and the cylinder are subjected to substantially sinusoidal motions which may be out of phase with the piston which is at lesser or greater amplitude, depending on the structure and operation conditions of the system. This will appear from the following:

LET:

y= The spacial amplitude of displacement of the cylinder at any part of the wave, for example at the angle 6 of the sinusoidal motion.

x= The spacial amplitude of displacement of the piston at said angle 6, due only to cyclic motion of the cylinder.

A The maximum displacement of the cylinder, i.e., the amplitude of the heave at the crest, which is one-half of the heave.

A The effective area of the piston, i.e., the sum of the effective area of the areas a of the piston 8 and b of the piston 13.

V, Volume of the gas in the accumulator when the system is at rest, i.e., when 0 0.

P The pressure in the gas on the liquid at V V The volume of the gas in the accumulator at any angle 0 of the cycle.

P The pressure in the accumulator when the volume is V 2 The spacial advance of the bit into the earth per second.

p The period of the cyclic motion in seconds.

w The contribution to the spacial displacement of the piston due to advance of the drill string into the earth as in drilling per degree of the cycle at any angle 0 of the cycle.

The piston motion is influenced by damping considerations and moves out of phase with the motion of the cylinder. The phase angle d: depends on the dynamics of the system.

It has been observed that the cylinder motion with respect to the space is a cyclic motion such as a sinusoidal motion. The spacial displacement of the cylinder at any angle 6 of the sinusoidal motion may be expressed as y A Sin Band the spacial displacement of the piston per degree of the cycle at the angle of the cylinder cycle due only to the cyclic action is X A f where f (6) is a function of the damping and other conditions of the system which may vary from cycle to cycle and even during any cycle and for wave condition, i.e., frequency and amplitude.

The relative displacement of the cylinder and piston,

assuming no advance of the drill occurs, is, keeping the direction signs of the motion in mind,

yix

The change in volume AV of the liquid and of the gas, the total liquid volume in the cylinder and accumulator being constant is iAV A, (y i x) If, however, the piston descends due to the advance of the drill string, the total advanced dgo per quarter cycle is The change in liquid volume in the cylinder and the change in gas volume in the accumulator AV=A (yixw) Since at the end of each quarter cycle y i A the net relative displacement of the piston and cylinder is At the end of each quarter of the cycle the change AV is The pressure in the cylinder, however, will be a function of the residual volume i.e.,

and in the case of an advancing drill, by making We do this by adding or subtracting from the liquid under the piston and in the accumulator an amount to maintain a constant volume of liquid and gas in the accumulator.

The following data may be taken as representative of possible practical conditions: P 2500.00 psig, i.e., 180,000 lbs. of force leaving 20,000 lbs. on the bit. Let A, (a b), the effective areas, where a equals 0.45 sq. ft. and b equals 0.05 sq.ft. The travel of the piston is taken as 5 ft. in each quarter of the cycle, i.e., half the heave of 10 feet, the period of the heave is 10 seconds, and z 0.02 feet per second, i.e., 72 feet per hour. Assume a stroke of 30+ feet so that the piston shall travel a distance of one stand of drill pipe before bottoming on the bottom of the cylinder and stroke 5 seconds per 10 foot heave. For purposes of the illustration, we will assume n is 1.

In the first quarter at the crest, i.e., 0 the piston has traveled for 2.5 seconds and has descended 0.05 feet. The cylinder has risen 5 feet. Therefore, the change in volume which has occurred is:

AV= 0.5 (5 0.05) 2.525 cu. ft.

The original volume of cu. ft. of gas in the accumulator at the start of the heave is reduced to 97.475 cu. ft. and a pressure of 2,565 lbs.

Since this is a decrease in volume of liquid in the cylinder, this is a volume which has to be removed from the system in order that it not be transferred to the pneumatic accumulator and thus not change the volume of gasin the accumulator. During the second and third quarter, when the cylinder is moving from crest to trough and the piston is advancing during drilling while the cylinder is moving from crest to trough, the piston advances 0.05 feet in each quarter cycle, i.e., during the second and third quarter, will each result in an increase of liquid volume in the cylinder, a decrease of volume of liquid in the accumulator and an increase of volume of gas V= 0.5 (5 0.05) 2.475 or 4.95 cu. ft. during the two quarters. The volume of 97.475 cu. ft. of gas in the accumulator would be increased to 102.425 cu. ft. and a pressure of 2,441 psig at the trough.

This is the amount of fluid which must be added to the hydraulic system in order that the volume of liquid in the accumulator remain constant. In the last quarter, in moving from the trough to the midpoint of the heave, the piston has descended another 0.05 feet; and the cylinder has moved up 5 feet in space; so the volume would be decreased:

V= 0.5 (5 0.05 feet) 2.525 cu. ft.

This is a decrease in gas volume from 102.425 to 99.9 cu. ft. at 0 360, at the end of the first cycle. The pressure which results would be 2,503 psig equivalent to a force on the piston of 180,216 lbs. In order to hold the force to 180,000, 0.1 cu. ft. of liquid is withdrawn.

The net liquid volume removed per cycle from the hydraulic and pneumatic system is thus 0.1 cu. ft. per cycle to permit the drill to advance 0.2 feet/cycle while maintaining a constant volume of liquid in the pneumatic accumulator.

It is necessary to modulate the volume of the liquid to remove a greater volume of liquid during the ascending portion of the heave than is added during the descending portion of the heave. This is because the piston during the drilling moves in an opposite direction to the cylinder during the ascending portion of the heave and in the same direction during the descending portion of the heave.

If there is any' loss of liquid from the accumulator cylinder system, this loss is compensated for by adding the leakage volume to the system during the cycling. Since this leakage loss subtracts liquid from the accumulator, it may require a reduction in the withdrawal of fluid or even an addition of fluid during that portion of the cycle when fluid is to be withdrawn from the cycle and an increase in the addition of liquid when fluid is to be added to the pneumatic-hydraulic system.

The volume change which is required at any angle will be different as the value of 6 changes as will be apparent from what has been said above. The actual change in the volume resulting from the cyclic motion of the cylinder and the advance of the piston in space at any angle 0 of the cycle depends upon an unknown factor. This factor is the function of 0 which represents the spacial displacement of the piston due to cyclic action alone, and this depends upon the conditions of the system and other parameters which are known or may be readily ascertained.

In addition, the leakage factor may be an uncertainty. The effect on the force on the piston resulting from changes in volume in the accumulator resulting from the spacial advance of the cylinder due to heave, moditied by the drilling advance when this occurs and by leakage losses, when this occurs, is the integral of the effect of these factors on the force exerted on the piston. The signal which reports the change is thus an integration of these force changes.

In the above example the volume of liquid displaced in the assumed 30 foot stroke of the piston is cu. feet which at 0.2 feet per cycle pistonadvance will occur after 150 cycles i.e., at the end of 1,500 seconds or about minutes. At this point in time the drilling lines are adjusted to add a stand of drill pipe, the increase in weight is accounted for by adjusting the pressure in the accumulator and the operation is repeated.

in the above example, the piston has drilled off each cycle but a small fraction of the cylinder displacement. This is usually the case. Should, however, the drilling rate exceed the heave rate, then it may be that the liquid would have to be withdrawn during the entire period of the heave at a somewhat different rate than during the ascending than the descending portion of the heave as will be apparent to those skilled in the art for what has been said above.

As was explained above, all of the parameters which affect the volume changes in the cylinder during heave action, including the cyling of the cylinder, the spacial advance of the piston, the leakage are all integrated in the volume changes in the accumulator and signaled by changes in the force on the piston.

We employ these force changes to generate a signal which is responsive to these volume changes and modulate the volume in the accumulator to reduce the signal to substantially zero and thus cancel the volume changes which would otherwise occur and thus maintain a substantially constant volume of liquid in the accumulator and a substantially constant force on the piston whether the piston is to be maintained at a constant point in space or advance as when the drill string is drilling.

We claim:

I. In an apparatus adapted to be mounted on a vessel subjected to heave due to wave action, which apparatus includes a piston and hydraulic cylinder and co-acting pneumatic accumulator system for control of the pressure in the hydraulic cylinder, means for connecting a load to said piston, the improvement which comprises means adapted to pass liquid under pressure from a source of liquid to the accumulator system, during that portion only of the heave when the volume of the liquid in the accumulator tends to decrease, a reservoir, means for withdrawing liquid from the accumulator to saidreservoir during that portion only of the heave when the volume of liquid in the accumulator tends to increase.

2. In the apparatus of claim 1, control means for selectively opening communication between the accumulator and said source and selectively closing communication between the accumulator and said reservoir when said first mentioned communication is opened and control means for opening said second mentioned communication and closing said first mentioned communication.

3. In the apparatus of claim 2 said control means comprising a signal means responsive to the force imposed in said cylinder on the piston during all portions of the heave, said signal means including means to generate a signal responsive to the forces on the said piston, means to generate a signal responsive to the forces predetermined to be maintained on the piston, means to generate an error signal responsive to the differences between said first and second mentioned signals and means to selectively open and selectively close the aforesaid communications responsive to said error signal.

4. In the apparatus of claim 1, means adapted to suspend the said cylinder from drilling lines, pipe connection between the cylinder and-said accumulator, a gas connection to the accumulator connected to a source of gas under pressure, a pipe connection between said cylinder and said accumulator and said source, pipe connection between said accumulator and said reser voir, valves in each of said pipe connections, means to open and close the valves in one of said pipe connections and means to close and open the valve in the other of said pipe connections.

5. In the apparatus of claim 4 said means to open and close said valves, including means to sense the force on the piston in said cylinder, and said means to acting to open the valve in said pipe connection when the said force is substantially below a predetermined value and said means to open the valve on the pipe connection to said reservoir acting when the force exerted on said piston is substantially above said predetermined value.

6. In the apparatus of claim 5 said control means comprising a signal means responsive to the forces imposed on the piston in said cylinder, during all portions of the heave, said signal means including means to generate a signal responsive to the forces on the said piston, means to generate a signal responsive to the forces predetermined to be maintained on the piston, means to generate an error signal responsive to the differences between said first and second mentioned signals and means to selectively open and selectively close the aforesaid connection responsive to said error signal.

7. In combination with a drilling vessel, a derrick, drilling lines suspended from said derrick, a hydraulic cylinder suspended from said drilling lines, a piston in said cylinder, means to connect a load to said piston, pneumatic accumulator system for control of the pressure in the hydraulic cylinder, liquid reservoir, the improvement which comprises communication means adapted to pass liquid under pressure from the liquid reservoir to the accumulator system during that portion only of the heave when the volume of the liquid in the accumulator tends to decrease, and communication means for withdrawing liquid from the accumulator to said reservoir during that portion only of the heave when the volume of liquid in the accumulator tends to increase.

8. In the apparatus of claim 7, said communication means comprising a signal means responsive to the forces imposed on the piston in said cylinder, during all portions of the heave, said signal means including means to generate a signal responsive to the forces in said cylinder, means to generate a signal responsive to the pressure predetermined to be maintained in the cylinder, means to generate an error signal responsive to the differences between said first and secondmentioned signals and means to selectively open and selectively close the aforesaid communications responsive to said error signal.

9. In the apparatus of claim 8, means to suspend the said cylinder from the drilling lines, pipe connection between the cylinder at a point below the piston and said accumulator, a gas connection between the accumulator adapted to be connected to a source of gas under pressure, a pipe connection between the cylinder and accumulator and said reservoir; valves in each of said pipe connections and means to close and open the valves in the said pipe connections.

10. In the apparatus of claim 9, said means to open and close said valves, including means to sense the force on the piston in said cylinder on said piston, and said means to open the valve in said pipe connection to the said source operative to open said valve when the said sorce is substantially below a predetermined value and means to open the valve on the pipe connection to said reservoir when the force exerted on said piston is substantially above said predetermined value.

11. A method of controlling the spacial displacement of a piston rod which is mounted for vertical displacement on a vessel subject to wave action, a cylinder in which said piston is reciprocally mounted in said cylinder and in which said piston is subject to be loaded by a weight connected to said piston, which load is opposed by liquid under pressure in said cylinder under the said piston, communicated from a body of liquid under gas pressure, the steps of withdrawing liquid from said accumulator to the said body of liquid andto a body of liquid extraneous to the cylinder and accumulator maintained at a substantially lower pressure than the aforesaid gas pressure, when the volume in said cylinder under the piston is decreasing and adding liquid to said accumulator when the body of liquid in said cylinder is increasing whereby the liquid in said accumulator is maintained at a substantially constant volume.-

12. The method of claim 11, the step of advancing the piston spacially downward during each portion of the heave cycle and withdrawing from said accumulator to an extraneous point a quantity of liquid responsive to the said spacial advance.

13. In the process of claim 12, said withdrawal occurring only during that portion of the heave when the volume under the piston in the cylinder is decreasing.

14. In the process of claim 13, said liquid introduction into the accumulator of liquid being in quantity less than the quantity withdrawn, said introduction occurring only during that portion of the heave when said volume in said cylinder is increasing.

15. A method of drilling in submarine environment from a vessel subject to heave as a result of wave action which comprises suspending a drill string connected to a drill bit from a cylinder and piston, mounted on drilling lines suspended in a drill derrick, maintaining said piston under pressure from a pneumatic accumulator connected to said cylinder, advancing said drill string into the earth, continuously removing from said cylinder and accumulator during the period of said heave a volume of fluid displaced from said cylinder by the advance of said drill string into the earth while maintaining a substantially constant pressure in said cylinder during said advance and during said heave.

16. In the method of claim 15, said maintenance of substantially constant pressure comprising withdrawing liquid from the cylinder to a body of liquid under gas pressure in said accumulator during that portion of the heave when the volume of said cylinder under the piston is decreasing due to the heave and transferring liquid from the said body of liquid to the cylinder when the volume in the cylinder under the piston is increasing and withdrawing from said cylinder and body of liquid, avolume of liquid during each portion of said heave substantially equivalent to the volume of liquid displaced in said cylinder by the said advance, the volume of liquid in said cylinder and said body decreasing with time while maintaining the said pressure substantially constant.

PO-1O5O UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION EDWARD LARRALDE and GLEN ROBINSON Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 37, "resported" should be reported Column 7, line 39, "are" should be as Column 10, line 18, "the" should be this Column 11, line 58, "cyling" should be cycling Column 12, line 53 "to" should be omitted before "acting."

Column 13, line 2, insert pipe before "connection."

Column 13, lin3 44, "sorce" should be force Signed and Scaled this thirtieth Day of September1975 [SEAL] Arrest.-

RUTH C. M A SON C. MARSHALL DANN Arrvsrmg Ojjlre Commissioner of Parents and Trudr'markx 

1. In an apparatus adapted to be mounted on a vessel subjected to heave due to wave action, which apparatus includes a piston and hydraulic cylinder and co-acting pneumatic accumulator system for control of the pressure in the hydraulic cylinder, means for connecting a load to said piston, the improvement which comprises means adapted to pass liquid under pressure from a source of liquid to the accumulator system, during that portion only of the heave when the volume of the liquid in the accumulator tends to decrease, a reservoir, means for withdrawing liquid from the accumulator to said reservoir during that portion only of the heave when the volume of liquid in the accumulator tends to increase.
 2. In the apparatus of claim 1, control means for selectively opening communication betwEen the accumulator and said source and selectively closing communication between the accumulator and said reservoir when said first mentioned communication is opened and control means for opening said second mentioned communication and closing said first mentioned communication.
 3. In the apparatus of claim 2 said control means comprising a signal means responsive to the force imposed in said cylinder on the piston during all portions of the heave, said signal means including means to generate a signal responsive to the forces on the said piston, means to generate a signal responsive to the forces predetermined to be maintained on the piston, means to generate an error signal responsive to the differences between said first and second mentioned signals and means to selectively open and selectively close the aforesaid communications responsive to said error signal.
 4. In the apparatus of claim 1, means adapted to suspend the said cylinder from drilling lines, pipe connection between the cylinder and said accumulator, a gas connection to the accumulator connected to a source of gas under pressure, a pipe connection between said cylinder and said accumulator and said source, pipe connection between said accumulator and said reservoir, valves in each of said pipe connections, means to open and close the valves in one of said pipe connections and means to close and open the valve in the other of said pipe connections.
 5. In the apparatus of claim 4 said means to open and close said valves, including means to sense the force on the piston in said cylinder, and said means to acting to open the valve in said pipe connection when the said force is substantially below a predetermined value and said means to open the valve on the pipe connection to said reservoir acting when the force exerted on said piston is substantially above said predetermined value.
 6. In the apparatus of claim 5 said control means comprising a signal means responsive to the forces imposed on the piston in said cylinder, during all portions of the heave, said signal means including means to generate a signal responsive to the forces on the said piston, means to generate a signal responsive to the forces predetermined to be maintained on the piston, means to generate an error signal responsive to the differences between said first and second mentioned signals and means to selectively open and selectively close the aforesaid connection responsive to said error signal.
 7. In combination with a drilling vessel, a derrick, drilling lines suspended from said derrick, a hydraulic cylinder suspended from said drilling lines, a piston in said cylinder, means to connect a load to said piston, pneumatic accumulator system for control of the pressure in the hydraulic cylinder, liquid reservoir, the improvement which comprises communication means adapted to pass liquid under pressure from the liquid reservoir to the accumulator system during that portion only of the heave when the volume of the liquid in the accumulator tends to decrease, and communication means for withdrawing liquid from the accumulator to said reservoir during that portion only of the heave when the volume of liquid in the accumulator tends to increase.
 8. In the apparatus of claim 7, said communication means comprising a signal means responsive to the forces imposed on the piston in said cylinder, during all portions of the heave, said signal means including means to generate a signal responsive to the forces in said cylinder, means to generate a signal responsive to the pressure predetermined to be maintained in the cylinder, means to generate an error signal responsive to the differences between said first and second-mentioned signals and means to selectively open and selectively close the aforesaid communications responsive to said error signal.
 9. In the apparatus of claim 8, means to suspend the said cylinder from the drilling lines, pipe connection between the cylinder at a point below the piston and said accumulator, a gas conNection between the accumulator adapted to be connected to a source of gas under pressure, a pipe connection between the cylinder and accumulator and said reservoir; valves in each of said pipe connections and means to close and open the valves in the said pipe connections.
 10. In the apparatus of claim 9, said means to open and close said valves, including means to sense the force on the piston in said cylinder on said piston, and said means to open the valve in said pipe connection to the said source operative to open said valve when the said sorce is substantially below a predetermined value and means to open the valve on the pipe connection to said reservoir when the force exerted on said piston is substantially above said predetermined value.
 11. A method of controlling the spacial displacement of a piston rod which is mounted for vertical displacement on a vessel subject to wave action, a cylinder in which said piston is reciprocally mounted in said cylinder and in which said piston is subject to be loaded by a weight connected to said piston, which load is opposed by liquid under pressure in said cylinder under the said piston, communicated from a body of liquid under gas pressure, the steps of withdrawing liquid from said accumulator to the said body of liquid and to a body of liquid extraneous to the cylinder and accumulator maintained at a substantially lower pressure than the aforesaid gas pressure, when the volume in said cylinder under the piston is decreasing and adding liquid to said accumulator when the body of liquid in said cylinder is increasing whereby the liquid in said accumulator is maintained at a substantially constant volume.
 12. The method of claim 11, the step of advancing the piston spacially downward during each portion of the heave cycle and withdrawing from said accumulator to an extraneous point a quantity of liquid responsive to the said spacial advance.
 13. In the process of claim 12, said withdrawal occurring only during that portion of the heave when the volume under the piston in the cylinder is decreasing.
 14. In the process of claim 13, said liquid introduction into the accumulator of liquid being in quantity less than the quantity withdrawn, said introduction occurring only during that portion of the heave when said volume in said cylinder is increasing.
 15. A method of drilling in submarine environment from a vessel subject to heave as a result of wave action which comprises suspending a drill string connected to a drill bit from a cylinder and piston, mounted on drilling lines suspended in a drill derrick, maintaining said piston under pressure from a pneumatic accumulator connected to said cylinder, advancing said drill string into the earth, continuously removing from said cylinder and accumulator during the period of said heave a volume of fluid displaced from said cylinder by the advance of said drill string into the earth while maintaining a substantially constant pressure in said cylinder during said advance and during said heave.
 16. In the method of claim 15, said maintenance of substantially constant pressure comprising withdrawing liquid from the cylinder to a body of liquid under gas pressure in said accumulator during that portion of the heave when the volume of said cylinder under the piston is decreasing due to the heave and transferring liquid from the said body of liquid to the cylinder when the volume in the cylinder under the piston is increasing and withdrawing from said cylinder and body of liquid, a volume of liquid during each portion of said heave substantially equivalent to the volume of liquid displaced in said cylinder by the said advance, the volume of liquid in said cylinder and said body decreasing with time while maintaining the said pressure substantially constant. 